In digital communications systems, especially today's highly integrated transceiver design, quadrature signals, commonly referred to as I/Q (In-phase/quadrature) signals, have been commonly used in a receiver or transmitter for up or down frequency conversion.
I/Q modulators and demodulators are widely used in digital communications systems. I/Q demodulators have been extensively discussed in the technical literature. See, for example, Behzad Razavi, “RF Microelectronics,” Prentice Hall (1998) and John G. Proakis, “Digital Communications,” McGraw-Hill (1995). Also, some U.S. patents have issued, related to the technology of I/Q modulation and demodulation: U.S. Pat. No. 5,974,306, entitled “Time-Share I/Q Mixer System With Distribution Switch Feeding In-Phase and Quadrature Polarity Inverters” to Hornak, et al.; U.S. Pat. No. 5,469,126, entitled “I/Q Modulator and I/Q Demodulator” to Murtojarvi; U.S. Pat. No. 6,560,449, entitled “Image-Rejection I/Q Demodulators” to Liu; U.S. Pat. No. 6,330,290, entitled “Digital I/Q Imbalance Compensation” to Glas. Examples of system applications that incorporate and standardize I/Q modulation and demodulation include the GSM (Global System for Mobile Communications), IS-136 (TDMA), IS-95 (CDMA), and IEEE 802.11 (wireless LAN). I/Q modulation and demodulation have also been proposed for use in the Bluetooth wireless communication systems.
As can be appreciated by those skilled in the art, the quadrature signals÷orthogonality directly affects the overall accuracy of digital signal's modulation and demodulation. It has thus been the goal of the transceiver designers to try to maintain sameness in amplitude and orthogonal in phase for the quadrature signals.
Conventionally, the solution has been to first take digital sampling of the signal, and then use powerful digital signal processing to calculate the error. The error is converted back to analog and fed back to the analog front end circuit (“AFE”) to do error correction. While such a method may accurately find the error and provide satisfactory orthogonality, its architecture and circuitry tend to become too complex, since the design of the AFE must be compatible with the DSP. Also, the conventional method would require multiple digital-analog converters (DAC) and analog-digital converter (ADC), thus greatly increasing the area and power consumption of the transceiver.
Reference is to FIG. 1, where a simplified block diagram of the conventional wireless receiver incorporating an I/Q demodulator is illustrated. The receiver includes an antenna 100 that receives a transmitted RF signal. The signal received by the antenna 100 is applied to a band-pass filter BPF 105, which filters out-of-band RF signals and rejects signals at frequencies other than the frequency of the desired RF carrier. The output of the BPF 105 is applied to a low-noise amplifier LNA 110, where the level of the input RF signal is raised sufficiently to effectively drive the receiver's mixer circuitry. The output from the LNA 110 is applied to the receiver's mixer/demodulator block, where a digital, frequency-synthesized local oscillator (LO) 125 is incorporated. The mixer/demodulator block also includes a quadrature demodulator, which includes I demodulator 130 and Q demodulator 120. As is well understood by those skilled in the art, an in-phase version of the LO signal is delivered to the I demodulator 130, while a quadrature (90° phase shifted) version of the LO signal is delivered to the Q demodulator 120. The outputs of the mixers 120, 130, after applied to their respective channel selection filters (CSF) 150, 140 and ADC 170, 160, constitute the demodulated I and Q signals 175, 165, respectively.
As can be appreciated by those skilled in the art, mismatch in I and Q channels will introduce frequency crosstalk, thus degrading the signal-to-noise ratio (SNR). I/Q mismatch has become inevitable due to state-of-the art semiconductor device design and fabrication inability to achieve “perfect” matching between devices, even devices on the same die.
Therefore, it is desirable to provide a simple and yet effective solution to address the amplitude and phase effects. It is also desirable for the solution to be combined with the high-frequency gain amplifier without increasing too much chip area.